Metamynodon
Metamynodon is an extinct genus of amynodont perissodactyls, and is among the longest lived genera of amynodonts, having first appeared during the late Eocene, and becoming extinct during the early Miocene, when it was supplanted by the semiaquatic rhinoceros, Teleoceras. Its fossils have been discovered in the United States (White River Fauna), Mongolia and China. Taxonomy Metamynodon is a member of the extinct family Amynodontidae, sometimes called "swamp rhinos" as they were once all believed to be semi-aquatic. It is split into two the tribes: the semi-aquatic Metamynodontini–''Paramynodon, ''Sellamynodon, Megalamynodon, and Metamynodon–and the tapir-like Cadurcodontini–''Procadurcodon, ''Zaisanamynodon, Cadurcodon, and Cadurcotherium. The Metamynodontini are found across the world, with Paramynodon from Myanmar, Sellamynodon from Romania, and Megalamynodon and Metamynodon from North America.2 Megalamynodon is likely the ancestor of Metamynodon, though it is possible it could have evolved from an Asiatic ancestor.3 In 1922, paleontologist Clive Forster-Cooper initially described tooth remains from Balochistan, Pakistan as M. bugtienses before returning 2 years later and reassigning it to the genus Paraceratherium.42 Metamynodon bones are common in Early Oligocene riverbed deposits of Badlands National Park in South Dakota, giving them the nickname "the Metamynodon channels."53 In 1981, a large jawbone, 41723-5, from Brewster County, Texas was described as M. mckinneyi, named after one of the discoverers Billy Pat McKinney, and was found in an area dated to 43 mya in the Late Eocene. The jawbone and tusks were more massive than that of M. chadronensis, and it could represent either a transitional phase between Amynodon and M. chadronensis, or simply a migrant to the area. Characteristics Metamynodon planifrons, the largest species, was about 4 metres (13 ft) in body length with a weight up to 1.99 tons, and, although it was distantly related to the modern rhinoceros, looked more like a hippopotamus.1 Its front legs had four toes instead of the three found in modern rhinos. Like other swamp rhinos, Metamynodon has large canines, and Metamynodonts generally have larger canines than other swamp rhinos. The shortened snout and large nostrils indicate Metamynodon had a prehensile lip. Metamynodonts were more brachycephalic and had a broader than longer snout. The nostrils would have been on the top portion of the snout like the modern hippo. As other aquatic mammals, Metamynodon had a poorer sense of smell than other swamp rhinos. The eyes were located high on the skull so it could see while also being completely submerged.83 Like other aquatic mammals, Metamynodon had small neural spines projecting upwards from the thoracic vertebrae, indicating weak neck muscles, which probably was due to buoyancy and a lack of necessity to support the head while submerged. The ribcage was broad and Metamynodon had a barrel-like chest similar to the hippo, which could either be an adaptation to an expanding digestive tract or to develop muscles necessary to prevent rolling over in the water. Like other aquatic mammals, the leg muscles popliteus in the knee, gastrocnemius in the calf, soleus in the calf, extensor digitorum longus in the foot, and peroneus tertius in the lower leg were all large and well-developed, probably to exert more force while walking and better wade through muddy soil. Although it was a herbivore (as indicated by its teeth), its skull had a bony ridge typically associated with carnivorous mammals. It probably fed on tough plant material, and the jaw muscles were attached to this ridge. Metamynodon would have used its enlarged canines to search for food in river banks, and may also have had highly flexible lips. Its eyes were placed high on the skull, meaning that it would have been able to see while almost fully submerged, much like a hippopotamus or crocodile. Paleobiology Category:Perissodactyla Category:Prehistoric mammals of Asia Category:Oligocene perissodactyls Category:White River Fauna